1. Technical Field
This invention relates to an asynchronous transfer mode (ATM) exchange system, and more particularly, relates to a circuit for calculating the number of idle virtual path identifiers and virtual channel identifiers in an ATM exchange system.
2. Related Art
Generally, ATM is a specific packet-oriented transfer mode using an asynchronous time division multiplexing technique where both line switching and packet switching are unified and many pieces of information such as voice, data, and pictures are organized in fixed-sized blocks, called cells. ATM can operate as both a packet exchange system and a circuit exchange system in one transmission mode for efficiently embodying both high speed and wide band communication networks. For this reason, ATM is a target transfer mode solution for implementing a broadband integrated services digital network (B-ISDN) whereas conventional synchronous transfer mode (STM) can only transmit subscribers information on channels having a fixed band.
Both ATM and STM have similarities, in that they transfer information in a digital format and support multiple channels for the flow of information. There is, however, one significant difference between the two systems relating to a method of assigning and identifying information channels. In STM, information is transferred through a fixed channel, where the channel is identified according to relative location of a frame pattern. If a channel is not transmitting information, the channel remains continuously assigned. Thus, in STM, the band channel is not effectively utilized. In ATM, on the other hand, a band channel is assigned only if there is information for transmission. Band channels that are not assigned can be used for information transmission by another subscriber. As a result, ATM offers a more effective channel management. Information channel identification in the ATM is embodied by the virtual path and virtual channel identifiers (hereinafter, referred to as "VPI & VCI") or similar connection identifiers which are placed in a header of each cell for identifying cells belonging to the same virtual channel on an asynchronous time division multiplex. Such channel identification of the ATM facilitates the use of a multiplex, de-multiplex and exchange of digital information, and serves to decrease the costs of broad-band communication network facilities because of its flexibility in a bandwidth distribution. Conventionally, there are a variety of ATM switching networks. Exemplary configurations are disclosed, for example, in U.S. Pat. No. 4,956,839 for ATM Switching System issued to Torii et al., U.S. Pat. No. 5,214,642 for ATM Switching System And Adaption Processing Apparatus issued to Kunimoto, U.S. Pat. No. 5,249,178 for Routing System Capable Of Effectively Processing Routing Information issued to Kurano et al., U.S. Pat. No. 5,280,475 for Traffic Shaping Method And Circuit issued to Yanagi et al., U.S. Pat. No. 5,280,483 for Traffic Control System For Asynchronous Transfer Mode Exchange issued to Kamoi et al., U.S. Pat. No. 5,301,184 for Control System For Switching Duplicated Switch Units In ATM Exchange issued to Uriu et al., U.S. Pat. No. 5,357,510 for Apparatus And A Method For Supervising And Controlling ATM Traffic issued to Norizuki, U.S. Pat. No. 5,537,402 for ATM Switch issued to Notani et al., and U.S. Pat. No. 5,557,609 for Switching Apparatus for ATM issued to Shobatake et al.
In a communication system or switching system using the ATM, a call processing unit must assign the VPI and VCI to each ATM cell whenever the subscriber requires a call connection. The VPI and VCI assignment is accomplished by reading in the order of VPI and VCI which are not under assignment every time the subscriber requires a call connection as disclosed, for example, in U.S. Pat. No. 5,119,369 for "Packet Switch Communication Network Using Packet Having Virtual Channel Identifier" issued to Tanabe et alii. A call which receives the assignment of the VPI and VCI placed in a header area of an ATM cell is then transferred through a transferring channel. Assigning VPI and VCI for call connection is, however, extremely burdensome to the call processing unit. This is because the call processing unit must search for available VPI and VCI values whenever a call is made. At an initial stage of assignment, it may be easy to sequentially allot values to VPI and VCI responding to the call connection requirement. After assigning values to all VPIs and VCIs to be utilized, however, the call processing unit must check for idle VPI and VCI. When there is a call, the ATM network must manage the number of VPI and VCI, which are used for connecting a call, and the number of idle VPI and VCI available.
Conventional ATM switching system includes a VPI and VCI table for indicating the status of each VPI and VCI. Busy VPI and VCI are counted to check how many VPI and VCI are in use. Idle VPIs and VCIs are also counted. Busy VPIs and VCIs, and idle VPIs and VCIs are managed within the table. The status of VPI and VCI are monitored by counting busy VPIs and VCIs and idle VPIs and VCIs where necessary. Generally, there are few problems in counting busy and idle VPIs and VCIs when a small number of VPIs and VCIs are utilized. However, as the number of VPIs and VCIs utilized increases, it becomes more difficult to count busy and idle VPIs and VCIs. Thus, it takes longer to perform this operation, and management cannot be performed efficiently.